The present invention relates to improvements in interlocking or interconnecting helical guide and advancement structures such as helical flanges and, more particularly, to mating helical flange arrangements having an anti-splay lip on one flange and a cooperating and interlocking anti-splay groove on the other flange, the flanges being configured so that when radial loading or engagement occurs, the lip and groove resist splaying of an outer one of the members having one of the cooperating flanges on it. Such flanges with anti-splay contours are particularly advantageous when used in combination with open headed bone screws formed with extended arms or tabs to facilitate the capture and reduction of spinal fixation rods, after which the arm extensions or tabs are broken off at weakened areas to from a low profile implant. In the present invention, the interlocking anti-splay components are also found on the extensions such that force can be applied to a closure and through the closure to a rod positioned between the extensions without splaying the extensions, since the closure holds them in fixed position relative to each other as the closure traverses between the extensions.
Medical implants present a number of problems to both surgeons installing implants and to engineers designing them. It is always desirable to have an implant that is strong and unlikely to fail or break during usage. Further, if one of a set of cooperating components is likely to fail during an implant procedure, it is desirable to control which particular component fails and the manner in which it fails, to avoid injury and to minimize surgery to replace or repair the failed component. It is also desirable for the implant to be as small and lightweight as possible so that it is less intrusive to the patient. These are normally conflicting goals, and often difficult to resolve.
One type of implant presents special problems. In particular, spinal anchors such as monoaxial and polyaxial bone screws, hooks, and the like are used in many types of back surgery for repair of problems and deformities of the spine due to injury, developmental abnormalities, disease or congenital defects. For example, spinal bone screws typically have one end that threads into a vertebra and a head at an opposite end. The head is formed with an opening to receive a rod or rod-like member which is then both captured in the channel and locked in the head to prevent relative movement between the various elements subsequent to installation.
A particularly useful type of head for such above referenced bone screws is an open head wherein an open, generally U-shaped channel is formed in the head, and the rod is simply laid in the open channel. The channel is then closed with some type of a closure member which engages the walls or arms forming the head and clamps the rod in place within the channel. While the open headed devices are often necessary and preferred for usage, there is a significant problem associated with them. The open headed devices conventionally have two upstanding arms that are on opposite sides of the channel that receives the rod member. The top of the channel is closed by a closure member after the rod member is placed in the channel. Many open headed implants are closed by plugs that screw into threads formed on internal surfaces between the arms, because such configurations have low profiles.
However, such threaded plugs have encountered problems in that they produce radially outward forces that lead to splaying of the arms or at least do not prevent splaying that in turn may lead to loosening of parts and failure of the implant. In order to lock the rod member in place, a significant force must be exerted on the relatively small plug or on a set screw of some type. The forces are required to provide enough torque to insure that the rod member is clamped or locked securely in place relative to the bone screw, so that the rod does not move axially or rotationally therein. This typically requires torques on the order of 100 to 125 inch-pounds.
Because open headed implants such as bone screws, hooks and the like are relatively small, the arms that extend upwardly at the head can be spread by radially outwardly directed forces in response to the application of the substantial torquing force required to clamp the rod member. Historically, early closures were simple plugs that were threaded with V-shaped threads and which screwed into mating threads on the inside of each of the arms. The outward flexure of the arms of the head is caused by mutual camming action of the V-shaped threads of the plug and head as advancement of the plug is resisted by clamping engagement with the rod while rotational urging of the plug continues. If the arms are sufficiently spread, they can allow the threads to loosen, slip, or even disengage and the closure to fail. To counter this, various engineering techniques were applied to the head to increase its resistance to the spreading force. For example, the arms were significantly strengthened by increasing the width of the arms by many times. This is undesirable since it leads to a larger profile implant which is always undesirable and may limit the working space available to the surgeon during implant procedures. Alternatively, external caps were devised which engaged external surfaces of the head. In either case, the unfortunate effect was to substantially increase the bulk, size, and profile of the implant, especially when external nuts are used which may take up so much space along the rod as to leave too little space for all the implants needed.
The radial expansion problem of V-threads has been recognized in various other applications of threaded joints. To overcome this problem, so-called “buttress” threadforms were developed. In a buttress thread, the trailing or thrust surface, also known as the load flank, is oriented perpendicular to the thread axis, while the leading or clearance surface, also known as the stab flank, remains angled. This results in a neutral radial reaction of a threaded receptacle to torque on the threaded member received. However, even buttress threaded closures may fail since they do not structurally resist splaying of the arms. The same is true for square threads that are sometimes used on closures of open headed implants.
Development of threadforms proceeded by applicant from buttress and square threadforms, which have a neutral radial effect on the screw receptacle, to reverse angled threadforms which can positively draw the threads of the receptacle radially inward toward the thread axis when the plug is torqued. In a reverse angle threadform, the trailing side of the external thread is angled toward the thread axis instead of away from the thread axis, as in conventional V-threads and provide an interference fit. However, outward radial forces on the arms at higher torques can lead to slipping from an interference fit. A positive mechanically interlocking structure between the arms and the closure is more desirable and structurally secure. In the present invention, such positive interlocking is also provided in vertical extensions of the arms that are eventually broken away and removed.
When rods are used in spinal fixation systems, it is often necessary to shape the rod in various ways to properly position vertebrae into which open headed bone screws have been implanted. The bone screw or implant heads are minimized in length and height to thereby minimize the impact of the implanted system on the patient. However, it is often difficult to capture a portion of a straight or curved rod in a short implant head to clamp it within the arms. The extensions allow the arms to extend upwardly and capture the rod therebetween. In this way, the closure can be more easily inserted and rotated to drive the rod downwardly into the head of the implant and reduce or realign a vertebra up to the rod.